(1) Field of the Invention
The present invention relates to a composite vessel lid comprising an outer lid and an inner lid in combination.
(2) Description of the Related Art
A composite vessel lid comprising an outer lid including an annular top panel wall having an opening in the central portion, a cylindrical skirt hanging down from the peripheral edge of the top panel wall, holding projections formed on the inner circumferential surface of the skirt and a screwed portion formed in the lower portion of the outer lid and adapted to engage with a neck of a vessel, and an inner lid of a thin metal sheet held on the inner side of the outer lid by said holding projections is known.
In the composite vessel lid this type, the outer lid in which the screwed portion is formed is composed of a plastic material, and the inner lid is composed of a metal. Accordingly, this composite vessel has merits of both the metal cup and plastic cup.
However, problems arise when this vessel lid used for sealing a content having an autogeneous pressure, such as a carbonated drink.
More specifically, in a vessel lid used for sealing a content having an autogeneous pressure, such as a carbonated drink, if the inner pressure is not released at the initial stage of opening, spouting of the content is caused at the time of opening, or the vessel lid per se is blown off. This trouble is especially conspicuous in case of a lid having a screwed portion composed of a plastic material.
For example, in case of a combination of a vessel lid formed of a metal and a glass bottle, screws of both the members have a rigidity, and even at the initial stage of opening, a vent passage for releasing the inner pressure through the space between the screws is easily formed. However, if the screwed portion is formed of a plastic material, since the screw per se has certain compressibility and deformability, even at the stage where opening is advanced to some extent by turning of the vessel lid, air tightness is still maintained between the screw of the lid and the screw of the vessel and a vent passage is hardly formed.
As means for solving this problem, there is known a method in which a vent hole is formed in a side wall between a sealed portion and a screw for releasing the inner pressure at the initial stage of opening (see Japanese Patent Publication No. 30266/81).
This vent hole can formed relatively easily in case of a metal lid having excellent processability and machinability, but in the case where the screwed portion is formed of a plastic material, formation of a vent hole after the molding is difficult, and if it is intended to form a vent passage simultaneously with the molding, removal of the formed lid from a mold becomes difficult. Moreover, the strength of the screwed portion is reduced.